In the construction sector, in general mechanical engineering and in electrical engineering, among other sectors, there is a demand for steels or alloys that feature a particular combination of mechanical and corrosion-chemical properties. They are often expected to have, at the same time, a high yield strength, good toughness, high fatigue resistance, high corrosion resistance and high wear resistance.
In crane and mobile crane construction, steels having a minimum yield strength of up to 1100 MPa are currently being used. Continuous further development of the high-strength fine grain construction steels is enabling an evolution in mobile crane construction as a result of the constant increase in load-bearing capacity with simultaneous reduction of service weight. Advances in mobile crane construction technology are increasingly requiring the provision of high-strength steel plate having a minimum yield strength of 1300 MPa.
The prior art discloses hot-rolled steel sheets characterized by good processibility and high tensile strength. Even when the tensile strength exceeds a particular value, for example 1200 MPa, delayed fracture of the steel plate may be caused. Such a fracture can be caused under the influence of a corrosion reaction that occurs in the steel sheet over the course of time, by virtue of hydrogen penetrating into the interior of the steel sheet. Consequently, in spite of its high tensile strength, such a steel sheet has a defect. Steel sheets having a high yield strength up to 1300 MPa accordingly require high resistance to such a delayed fracture.
Steel sheets having a high tensile strength or high minimum yield strength often have the disadvantage that they are processible by cold forming only with difficulty because of their poorer formability. Furthermore, steel sheets having a high tensile strength and high minimum yield strength often have poor toughness properties. Especially at low temperatures of −40° C. or lower, these steels have such low toughness values that use for construction machinery, which has to meet high toughness requirements at low temperatures, is impossible.
EP 2 267 177 A1 discloses a high-strength steel sheet which is used as a structural element in industrial machinery and which firstly has excellent resistance to delayed fracture and secondly has good welding characteristics. The steel sheet of the invention has a high minimum yield strength equal to or higher than 1300 MPa and a tensile strength equal to or higher than 1400 MPa. The thickness of the steel sheet of the invention is equal to or greater than 4.5 mm and less than or equal to 25 mm.
However, the steels which are described in the prior art are not satisfactory in every aspect, and there is a need for steels having improved properties.
Thus a need exists for a high-strength steel having a high minimum yield strength, high tensile strength and, at the same time, good cold forming characteristics and good toughness properties at low temperatures.